Preheat oven to 350˚F. Arrange the cubed challah in a single layer on 3 large rimmed baking sheets, and bake for 15 minutes, until golden brown. Allow to cool, then transfer to a large mixing bowl. Add the chestnuts to the bowl of challah. Turn the oven to 400˚F.

In a large skillet over medium heat, melt the butter. Stir in the celery (ribs and leaves), onions, and parsley. Cook, stirring occasionally, until softened, about 20 minutes. Transfer to the mixing bowl with the challah. Fold in the apples, thyme, and sage, and toss well to incorporate.

In a medium mixing bowl, whisk together the eggs and 2 cups of the stock or broth. Whisk in 1 tbsp. of salt and ½ tsp. of pepper. Pour over the challah mixture, and mix well. Add an additional ½ cup to 1 cup of stock; you’ll want the stuffing to be moist, but the bread cubes should remain intact.

Butter 2 large, shallow baking dishes, and spread out the stuffing in them. Bake stuffing uncovered for 20 minutes, or until heated through and lightly crisp on top.